SQLite 3.39

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CREATE TABLE pc
(
  c INTEGER NOT NULL
) STRICT;
✓
CREATE INDEX ipc
  ON pc (
    c DESC
  );
✓
INSERT INTO
  pc(c)
VALUES
  (8),
  (6), 
  (4), 
  (2),
  (0);
✓
SELECT * FROM pc


c


8

6

4

2

0


WITH RECURSIVE 
  enumerated (
    id,
    c
  )
AS
(
  SELECT
    ROW_NUMBER() OVER (ORDER BY c DESC)   id,
    c
  FROM
    pc
),
  meet_in_middle (
    c_id,
    c_val, 
    iterated_calc
  )
AS
(
  -- Anchor dataset with a dummy row, in case `pc` is empty, 
  -- and the first iterative calculation value
  VALUES (0, 0, 0)

  UNION ALL
  
  SELECT
    -- next row of `pc`
    next.*,
    -- next calculated value
    prev.iterated_calc + 1 
  FROM
    meet_in_middle   AS prev
  INNER JOIN
    enumerated       AS next
      ON next.id = prev.c_id + 1
      -- The id of the next `pc` row
      -- Similarity to iteratively calculated value is coincidental
  WHERE
    prev.iterated_calc <= next.c
    -- Only iterate if the previous iterated calculation
    -- does NOT satisfy our condition
)
SELECT
  iterated_calc
FROM
  meet_in_middle
ORDER BY
  c_id DESC
LIMIT
  1
-- Get only the last row from the CTE
;


iterated_calc


3